Sphingosine-1-phosphate (S1P) is part of the sphingolipid class of molecules. S1P is a bioactive sphingolipid that mediates a wide variety of cellular responses, such as proliferation, autophagy, blockade of apoptosis, cell differentiation, blockade of cell senescence, cytoskeletal organization and migration, adherence- and tight junction assembly, and morphogenesis. Moreover, S1P is a modulator of APP processing via BACE1 regulation as well as lipid raft formation and can interact with ABC transporters thereby modulating cellular in- and efflux. S1P can bind with members of the endothelial cell differentiation gene family (EDG receptors) of plasma membrane-localized G protein-coupled receptors. To date, five members of this family have been identified as S1P receptors in different cell types, S1P1 (EDG-1), S1P2 (EDG-5), S1P3 (EDG-3), S1P4 (EDG-6) and S1P5 (EDG-8). S1P can produce cytoskeletal re-arrangements in many cell types to regulate immune cell trafficking, vascular homeostasis and cell communication in the central nervous system (CNS) and in peripheral organ systems. The above mentioned actions of S1P are mediated by interaction with its receptors. Therefore, S1P receptors are therapeutic targets for the treatment of, for example, neoplastic diseases, diseases of the central and peripheral nervous system, autoimmune disorders and tissue rejection in transplantation.
It is known that S1P is secreted by vascular endothelium and is present in blood at concentrations of 200-900 nanomolar and is bound by albumin and other plasma proteins. This provides both a stable reservoir in extracellular fluids and efficient delivery to high-affinity cell-surface receptors. S1P binds with low nanomolar affinity to the five receptors S1P1-5. In addition, platelets also contain S1P and may be locally released to cause e.g. vasoconstriction. The receptor subtypes S1P1, S1P2 and S1P3 are widely expressed and represent dominant receptors in the cardiovascular system. Further, S1P1 is also a receptor on lymphocytes. S1P4 receptors are almost exclusively in the haematopoietic and lymphoid system. S1P5 is primarily (though not exclusively) expressed in central nervous system (CNS; brain and spinal cord). Other tissues with S1P5 expression are skin and spleen. Moreover, S1P5 is expressed on NK cells. Early study showed that the CNS expression in mice appeared restricted to oligodendrocytes, while in men and rats expression was more diverse. Recent evidence has shown a broader distribution in all species: S1P5 expression is shown at the level of astrocytes, endothelial cells, glial cells, oligodendrocytes and to a lesser extent neurons.
The present invention relates to modulators of the S1P5 receptor, in particular agonists, and preferably to agonists with selectivity over S1P1, S1P3 and/or S1P4 receptors, in view of unwanted cardiovascular and/or peripheral immune-modulatory effects. It has now been found that S1P5 agonists can be used in the treatment of cognitive disorders, in particular age-related cognitive decline. Moreover, evidence has shown an impact on amyloid β (protein) processing, ABC transporter expression, blood-brain-barrier integrity, neuro-inflammatory processes, and (sphingo)lipid content in the CNS.
The latter is of high relevance as an altered sphingolipid metabolism is strongly implicated in several neurodegenerative and cognitive diseases. A comparison of CNS gene expression profiles of normal and Alzheimer's Disease (AD) patients indicated that genes responsible for SLP degradation were strongly upregulated, including the phosphatidic acid phosphatase PPAP2A and S1P lyase genes, while genes for ceramide production (apoptotic sphingolipid) were upregulated (Katsel et al, 2007, Neurochem Res, 32, 845-856). These gene expression data are predictive of actual changes in enzyme and lipid levels in the brain and cerebrospinal fluid (CSF): compared to normal subjects, AD brain are characterized by higher levels of ceramide and cholesterol as well as decreased levels of S1P. These changes also correlate with disease severity of the patients and are related to levels of Amyloid β and Tau, two hallmarks of Alzheimer's Disease (Cutler et al, 2004, PNAS, 101, 2070-2075; He et al, 2010, Neurobiol. Aging, 31, 398-408; Koal et al 2015. J. Alz Disease, 44, 1193-1201). The same changes have been reported in brain tissues (and CSF) from patients suffering HIV dementia, Amyotrophic Lateral Sclerosis (ALS), Parkinson's Disease, Parkison's Disease with Lewy Bodies, Multiple Sclerosis, Huntington's Disease, and several sphingolipdidosis disorders (Lysosomal Storage Disorders) such as Niemann Pick Disease and Gauchers (Cutler et al, 2002, Ann Neurol, 52, 448-457; Haughey et al, 2004, Ann Neurol, 55, 257-267; Cutler et al, 2010, Neurol, 63, 636-630; Mielke et al, 2013, PLOS ONE, 8; Bras et al, 2008, FEBS Journal, 275, 5767-5773; Vidaurre et al, 2014, Brain, 137, 2271-2286; Fan et al, 2013, J Lipid Research, 54, 2800-2814). Modulating the activity of the S1P5 receptor in the central nervous system may be a therapeutic method for such neurodegenerative or cognitive disorders by shifting the ceramide/S1P balance towards S1P effects and away from ceramide-mediated cell death.
Soluble β-amyloid (Aβ) oligomers are considered the proximate effectors of synaptic injury and neuronal death occurring in AD. Aβ induces increased ceramide levels and oxidative stress in neuronal cultures, leading to apoptosis and cell death. S1P is a potent neuroprotective factor against this Aβ-induced damage, consistent with its role as ceramide's counterpart (Cutler et al. 2004, PNAS, 101, 2070-2075, Malaplate-Armand, 2006, Neurobiol. Dis, 23, 178-189). Aβ is also pro-inflammatory, inducing the migration of monocytes to sites of injury, and the S1P1, S1P3, S1P4, S1P5 agonist FTY720/Fingolimod inhibits such migration. Aβ is known to induce expression of S1P2 and S1P5, but not of S1P1, S1P3 and S1P4 (Kaneider et al, 2004, FASEB). The actions of FTY720/FIngolimod and those expressed by monocytes suggests these effects are mediated by the S1P5 receptor. The same applies to more recent findings that FTY720/Fingolimod is able to modulate Aβ-induced memory deficits (Fukumoto et al, 2014, Beh Brain Res, 268, 88-93).
Additional studies suggest a role for S1P in modulating pain signals. In example, S1P modulates action potentials in capsaicin-sensitive sensory neurons (Zhang et al, 2006, J Physiol, 575, 101-113) and S1P levels are known to be decreased in CSF in acute and inflammatory pain models (Coste et al, 2008, J Biol Chem, 283, 32442-32451). The S1P1, S1P3, S1P4, S1P5 receptor agonist FTY720/Fingolimod is indeed able to reduce nociceptive behavior in neuropathic pain models (Coste et al, 2008, 12, 995-1004), while the selective S1P1 agonist SEW2817 fails to have an effect. Given the high CNS expression of S1P5 and lack of effects of S1P1 agonism, the effects can be contributed to effects on the S1P5 receptor.
In summary, potent and selective agents that are agonists of the S1P5 receptor will be beneficial for the treatment of cognitive disorders, neurodegenerative disorders and pain. In particular, S1P5-selective ligands would be beneficial for these diseases by not engaging the S1P1, S1P3 and/or S1P4 receptor ensuring a lack of peripheral immune suppression and cardiovascular side-effects.
WO 2011/017561 describes S1P agonists containing a fused cyclic core wherein optionally one the rings is a heterocycle. The compounds therefore structurally differ from the compounds of the present invention.
WO 2012/004373 describes S1P receptor modulators containing a fused heterocyclic core. These fused heterocyclic core structurally differs from the compounds of the present invention in the size of the rings constituting the core and the type and number of heteroatoms present in the rings.
Currently, there is still a need for new, potent S1P receptor modulators, in particular selective S1P5 receptor modulators.